The present invention relates to the optical measurement of the dimensions, submicron or close to a micron, of an object and measurement of the amplitude amount of overlay of two superimposed objects or patterns, and more particularly a method and device for obtaining very high measurement accuracy, less than one tenth of a micron. The invention finds a particularly important, although not exclusive, application in the automatic control of the widths of lines on wafers carrying patterns of integrated circuits with larger scale integration, overlay of the thin layers forming said wafers and/or integrated circuit wafer mapping.
The technology of production of integrated circuits evolves rapidly and particularly the width of the lines of the integrated circuits does not cease to diminish. At the present time, integrated circuit production technologies make it possible to etch lines of about one micron, but these technologies are rapidly outstripped. Lines of 0.1 micron can already be obtained experimentally and the American SEMATEC program (Semiconductor Manufacturing Technology) foresees the development of a technology for producing static memories SRAM of 16 Mbits with line widths of 0.3 micron and wastage less than 3% for the beginning of the 1990s.
It then becomes imperative to improve, both from the accuracy and reliability points of view, the measurement of the critical dimensions of submicron lines. The objective in the accuracy of measurement of the critical dimensions for future technologies in the production of integrated circuits is about 0.01 micron. No process, no device of the prior art makes it possible to fulfil such specifications.
Similarly, the degrees of overlay between the patterns, which are normally superimposed, of two layers of integrated circuits must be controlled with better accuracy; in fact, measurements of the overlay of layers by optical vernier or by electronic control are not satisfactory, at least in certain stages of production of a wafer.